My Car Shakes Noticebly when I Put The Gear into Reverse When The Car Is on a Slope

[Meteorite2]

I have a Toyota Yaris with automatic transmission gearbox.

Today when I wanted to put the gear into reverse in order to reverse against an slope, the car made was an odd, loud sound and shook quite noticeably. Then I parked the car upwards, and tried to reveres, same result.

Please watch the following youtube shot I got to see for yourself what I mean.

https://www.youtube.com/watch?v=jSMwzYI3deY

From the video you can see how the car actually shakes.

It always happens for the first time I put the gear from P to R. After I put the gear back to P gear, one of the two scenarios below happens if I want to put it on R for the second time:

1- if I keep pushing the break pedal from the beginning until I want to put the gear back to R for the second time, car doesn't shake for the second time I put it on R. (no handbrake)

2- if I take my foot off the break after putting the gear back to P, then the car goes forward or backward a few centimeters depending whether I have parked the car on the slope upwards or downwards (no handbrake). Then, if I put the gear in R again, the car shakes once again.

Can anyone tell me if it's a serious problem?

Thanks guys,